This invention relates to the improved permanent magnet properties of ferromagnetic and ferrimagnetic MnBi, in the equilibrium, ferromagnetic phase (LTP) or non-equilibrium, ferrimagnetic phase (HC) produced in an aligned rod morphology by directional solidification. Moreover, this invention relates to MnBi rods which contain a high coercivity or HC phase which provides exceptionally high permanent magnet properties at cryogenic temperature and can serve as a magnetic switch. The aforesaid high coercivity or HC phase is found to coexist with the LTP phase.
The novel produced MnBi rods of this invention serve as permanent magnet material for temperatures in the range of -50.degree. C. -150.degree. C. and at cryogenic temperature. These permanent magnet properties are comparable to presently used sintered rare earth-cobalt permanent magnets and can be mass produced at reduced costs.
The permanent magnet (ferromagnetic) character of MnBi has long been established by a number of investigators. The MnBi intermetallic, which has a hexagonal NiAs crystal structure, possesses one of the highest magnetocrystalline anisotropies (K.sub.1 =9.1.times.10.sup.6 erg/cm.sup.3, K.sub.2 =2.6.times.10.sup.6 erg/cm.sup.3) and magnetostrictions (.lambda..sub.1 =-800.times.10.sup.-6) of any material other than the rare earth-cobalt alloys.
In order to optimize the permanent magnet figures of merit for this system, i.e., remanent induction, intrinsic coercivity and energy product: (1) the magnetic moment of each magnetic region or domain should be aligned with every other region such that each moment lies along the easy axis of magnetization and (2) the average domain size should be reduced below a critical size at which a single-domain structure is of lower energy (and hence more probable to exist) than a multi-domain structure and below which magnetization reversal by coherent rotation is favored over domain wall motion, reversed domain nucleation or incoherent curling processes.
However, the permanent magnet figures of merit may also be a sensitive function of the shape of the magnetic domain (shape anisotrophy), the elastic deformation of a domain under application of an external magnetic field (magnetostriction), local demagnetizing fields at surface irregularities or inclusions of the domains (strain), local elastic strains near dislocations and other lattice defects (magnetostriction, anti-ferromagnetic coupling, domain wall pinning), a local Bi-rich region produced by preferential oxidation of Mn (local reduction in magnetocrystalline anisotropy) or the coexistence of a second magnetic phase which could be ferromagnetic (magnetic coupling) or paramagnetic (ultra fine MnBi particles).
Previous attempts to produce single domain size, aligned permanent magnets using MnBi have involved hotpressing of ground powder in a very high external magnetic field which presumably aligns the easy axis of magnetization of different size and small MnBi particles (U.S. Pat. No. 2,825,670 to Adams et al). The coercive field for MnBi powder was found to be extremely sensitive to average particle size, varying from an intrinsic coercivity, .sub.m H.sub.c, of 600 Oe for 100 .mu.m diameter particles to 12000 Oe for particles less than 3 .mu.m in size (U.S. Pat. No. 2,579,679 to Guillaud). Unfortunately, due to the shape and deformation of the powder crystallites, permanent magnet properties achieved with MnBi powder are:
remanent induction (Br)=4300 gauss PA1 intrinsic coercivity (.sub.m H.sub.c)=3400 Oe PA1 maximum static energy product (BH).sub.max =4.3 MG-Oe at room temperature as compared with the theoretical maximum of: PA1 Br=8200 gauss PA1 .sub.m H.sub.c =35000 Oe PA1 (BH).sub.max =17 MG-Oe
We have used a different approach involving plane-front directional solidification to achieve aligned particles or rods of MnBi within a protective, non-ferromagnetic Bi matrix whose size is an order of magnitude less (&lt;0.5 .mu.m) than those obtained previously by grinding and crushing. This approach does not pertain to casting of nearly dense MnBi ingots (U.S. Pat. No. 2,804,415 to Boothby et al) to produce an ingot containing only the ferromagnetic (LTP) MnBi phase which by itself is not a suitable permanent magnet but requires additional processing such as grinding, sintering, etc. as outlined above.